Linear optics correction in the CEBAF accelerator

Abstract

During commissioning of the CEBM accelerator, correcting dispersion, momentum compaction and betatron beam envelopes was essential for robust operation. To speed the diagnostic process we developed a method which allows one to track and correct the machine optics on-line.The method is based on measuring the propagation of 30 Hz modulated betatron oscillations. The beam optics of the accelerator was altered to decrease lattice sensitivity at critical points and to simpli@ control of the betatron function match. The calculation of the Courant-Snyder invariant from signals of each pair of beam position monitors was used for a correction of the betatron functions. The experience of optics correction and the study of long and short term machine reproducibility obtained during 1996 and early 1997 are also discussed. With minor modifications this method can also be used for on-line optics measurement and correction in circular accelerators.

CBETA Design Report

“…Operational algorithms -such as those employed in CEBAF [38] -may be required to negotiate corrections amongst the various passes of beams through the system so as to avoid lattice error hypersensitivity, beam size blowup, or problems with halo and beam transmission.…”

Beam Diagnostics Based on AC Modulation of System Parameters

“…The resulting 30 Hz differential orbit variations are detected by the beam position monitor (BPM) system. These synchronous position variations are fitted in groups of from 3 to 5 bpms to determine (x,x',y,y') values for the beam centroid along the machine relative to the unperturbed beam [9]. The actual number of oscillations executed by a particle on its path is not of great importance.…”